Optical communications systems are coming into more and more widespread use because of the low transmission loss and low cost of optical fibers, and the extremely wide communications bandwidth afforded by them. Digital optical communications are widely used but analog techniques are still relatively undeveloped.
Optical analog intensity modulation operates by varying the intensity of a laser output in response to the analog modulating signal.
Until recently, analog signals were transmitted over an optical channel either by intensity modulation of the optical output of a diode laser or by using an A/D converter to convert the analog signal, sending the result as digital data over the optical channel, and reconverting the received digital data to an analog signal using a D/A converter. Intensity modulation is selected for analog signal communication whenever either or both the A/D dynamic range (number of bits) and/or the sampling rate are insufficient to provide the desired communication performance. The two means for generating an intensity modulated optical signal are direct modulation of the injection current of a diode laser and the use of an external integrated optics Mach-Zender interferometer.
During the last several years a technique known as subcarrier FM has been introduced. In this scheme the analog signal to be transmitted is used to modulate an electronic voltage controlled oscillator (VCO). The VCO output, with constant amplitude and varying frequency, is converted to a square wave using a limiter. The resulting digital signal is transmitted over a digital fiber communications system. The detected electronic digital signal is then demodulated using a discriminator to yield the original analog signal.
The dynamic range of a communication system is determined by the system noise floor for small signals, and by the saturation level for large signals. In an amplitude (intensity) modulated system, the modulation/demodulation process does not provide any reduction of the noise introduced by the system components. The linearity of an AM system is determined primarily by the linearity of the device generating the modulation. In the case of both injection current modulation of the intensity of a diode laser optical output, or external modulation of the optical output of any type laser using a Mach-Zender interferometer, linearity is inherently limited by the modulator characteristics. The dynamic range of an AM system is thus limited by the noise produced by the components in the system and the linearity of the system components. The subcarrier FM system is limited by the modulation bandwidth of the VCO, typically less than 20 MHz, and the VCO modulation linearity.
Optical FM discriminators have been made using Fabry-Perot interferometers but their noise performance makes them unsuited for FM demodulation. Delay line discriminators are also known but these use very long differential delays (many tens of microseconds) for measuring laser linewidth or frequency noise.